1. Field of the Invention
The present invention relates to a display device, and more particularly, to a dual panel type organic electroluminescence device and method for fabricating the same.
2. Discussion of the Related Art
In general, an organic electro luminescence device, which also is referred to as an organic light emitting diode (OLED) device, is a self-emission type display and includes a plurality of pixels and an organic light emitting diode in each of the pixels. Each of the organic light emitting diodes emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Accordingly, the organic electro luminescence does not require an additional light source and has a light weight, thin profile, and compact size.
In addition, the organic electro luminescence generally is manufactured using a relatively simple process including a deposition process and an encapsulation process. Thus, an organic electro luminescence has a low production cost. Further, the organic electro luminescence can operate using a low DC voltage, thereby having low power consumption and fast response time. The organic electro luminescence also has a wide viewing angle, and high image contrast. Moreover, the organic electro luminescence is an integrated device. Thus, the organic electro luminescence has high endurance from external impacts and a wide range of applications.
A passive matrix type organic electro luminescence that does not have a switching element has been widely used. In the passive matrix type organic electro luminescence, scan lines intersect signal lines to define a matrix shape device, and the scan lines are sequentially driven to excite each pixel. However, to achieve a required mean luminescence, a moment luminance needs to be as high as the luminance obtained by multiplying the mean luminescence by the number of lines.
There also exists an active matrix type organic electro luminescence, which includes thin film transistors as switching elements within each pixel. The voltage applied to the pixels are charged in a storage capacitor Cst so that the voltage can be applied until a next frame signal is applied, thereby continuously driving the organic electro luminescence, regardless of the number of gate lines, until a picture of images is finished. Accordingly, the active matrix type organic electro luminescence provides uniform luminescent, even when a low current is applied.
FIG. 1 is a schematic view of a pixel of an active matrix type organic electro luminescence device according to the related art. In FIG. 1, a scan line 2 is formed along a first direction, and a signal line 3 is formed along a second direction intersected with the scan line 2, thereby defining a pixel region. A power line also is formed along the second direction and spaced apart from the signal line 3. A switching thin film transistor 5 in the pixel region, and a storage capacitor (CST) 6 is connected between the switching thin film transistor 5 and the power supply line 4. A driving thin film transistor 7, which is a current source element, is connected to the storage capacitor 6 and the power supply line 4.
In addition, an organic electro luminescence diode 8 is connected to the driving TFT 7. When a current is applied to an organic light emitting material of the organic electro luminescence diode 8 in a forward direction, electrons and holes are recombined, moving through a P-N junction between an anode electrode as a hole donor and a cathode electrode as an electron donor. Therefore, the energy of the organic electro luminescence diode 8 becomes lower, thereby generating an energy difference and causing light emission.
The organic electro luminescence may be classified into a top emission type and a bottom emission type based on its light emission direction. FIG. 2 is a schematic sectional view of a bottom emission type organic electro luminescence device according to the related art. In FIG. 2, an organic electro luminescence device 10 includes a first transparent substrate 1. A thin film transistor T, a first electrode 12, an organic luminescent layer 14 and a second electrode 16 are formed on the first substrate 1. The first and second electrodes 12 and 16 form an electric field. If the first electrode 12 is an anode electrode and the second electrode 16 is a cathode electrode, the first electrode 12 includes a transparent conductive material and the second electrode 16 includes a metal material having a low work function. The organic luminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c including one of red, green and blue organic luminescence material R, G, B to corresponding to a sub-pixel, and an electron transporting layer 14d. 
In addition, the first substrate 1 is adhered to a second substrate 30 by a sealant 40. The first and second substrates 1 and 30 are formed of a transparent insulating material, such as glass or plastic. The second electrode 16 and the second substrate 30 are separated by a distance. A desiccant (not shown) for absorbing moisture from an external and a semi-transparent tape (not shown) for attaching the desiccant to the second substrate 30 are included in the second substrate 30.
FIG. 3 is a detailed sectional view of a sub-pixel region shown in FIG. 2. As shown in FIG. 3, a semiconductor layer 62, a gate electrode 68, source and drain electrodes 80 and 82 are sequentially formed on the first substrate 1 in a thin film transistor portion. A power electrode 72 and an organic electro luminescence diode (E) are respectively connected to the source and drain electrodes 80 and 82. The power electrode 72 connects to a power supply line (not shown), and overlaps a capacitor electrode 64 with an insulator therebetween in a storage capacitor portion. Elements provided at the thin film transistor region and the storage capacitor region, except the organic electro luminescence diode E, form an array device A. The organic electro luminescence diode E includes the first electrode 12 and the second electrode 16 facing each other and sandwiching the organic electro luminescence layer 14. The organic electro luminescence diode E is disposed at an emission region where self-emitting light is emitted outside.
FIG. 4 is a flow chart illustrating a fabrication method of an organic electro luminescence device according to the related art. As shown in FIG. 4, at step ST1, an array device is formed on a first substrate. Forming the array device includes forming a scan line, a signal line, a power line, a switching thin film transistor and a driving thin film transistor.
Then, at step ST2, a first electrode is formed. Forming the first electrode includes connecting the first electrode to the driving thin film transistor. In addition, at step ST3, an organic electro luminescence layer is formed. When the first electrode is an anode electrode, forming the organic electro luminescence layer includes forming, in sequence, a hole injection layer, a hole transporting layer, an emission layer and an electron transporting layer.
At step ST4, the second electrode is formed. The second electrode is formed on the entire surface of the substrate as a common electrode. Then, at step ST5, the first substrate is encapsulated with a second substrate to protect the first substrate from an external impact and to prevent the organic electro luminescence layer from being damaged by an introduction of an external air. A desiccant is included in the second substrate.
However, the organic electro luminescence device according to the related art has a drawback. Since a yield of the array device is multiplied by a yield of the organic electro luminescence diode to determine an overall yield of the organic electro luminescence device, a total yield is greatly limited by a fabrication process of an organic electro luminescence diode. For example, even if the array device is formed with a good quality, the organic electro luminescence device is determined as a failure when the organic electro luminescence layer having a thickness of about 1000 Å is degraded due to a foreign material. As a result, the process and material costs in fabricating the good array device is wasted, and an overall production yield is decreased.
In addition, the bottom emission type device has a limited aperture ratio, thereby limiting its application in high-resolution displays. Thus, a top emission type device is more advantageous because it has an easy design of the thin film transistor and an improved aperture ratio. However, since the top emission type device generally has the cathode electrode disposed on the organic electro luminescence layer, there are limited choices in material for forming the cathode electrode. Thus, the transmittance of the top emission type device is limited by this narrow material selection range, thereby reducing light efficiency. Further, when a thin film type passivation film is used for minimizing a reduction in the light transmittance, external air is not sufficiently cut off from the interior of the top emission type device. Moreover, a problem exists in placing a desiccant in a top emission type device because the array device and the organic electro luminescence layer are formed on different substrates.